Project Summary Alopecia areata (AA) is one of the most common autoimmune diseases resulting in disfiguring hair loss, and carries significant psychosocial morbidity especially in children and adolescents. Since there are no FDA approved treatments for AA, the identification of targeted therapies with an improved safety profile would be a substantial advance. We recently identified CD8+NKG2D+ T cells as the key pathogenic cells in AA guiding our efforts to identify inflammatory cytokines that drive their activation and function. Our gene transcriptional profiling data showed that interleukin-7 (IL-7) as well as its receptor (IL-7R?) are upregulated in AA lesional skin. IL-7 is a cytokine essential for lymphocyte development and survival. Although IL-7 and its receptor have been implicated in several other autoimmune diseases, the role of IL-7 in the pathogenesis of AA is unknown. Our preliminary data show that IL-7 plays a critical role in AA which was underscored by the therapeutic benefit of IL-7 blockade in C3H/HeJ mouse model of AA. Although the role of IL-7 has been well studied in development, homeostatic proliferation, and survival of T cells, the mechanism by which the IL-7/IL-7R signaling pathway influences disease settings and pathogenic T cell responses in AA, particularly in vivo, has not been fully addressed. In Specific Aim 1 of this proposal, we will assess the link between IL-7 and alopecic T cells in vivo using a retrogenic TCR system to induce spontaneous AA in wild type or IL-7 knockout background. Although the expression of IL-7 is upregulated in AA, little is known about the mechanisms that regulate skin IL-7 production in AA. In Specific Aim 1, we postulate a feedback loop in which IFN-? is produced by hair follicle infiltrating T effector cells, which in turn promotes skin IL-7 production and the survival of T lymphocytes. Interestingly, we have found that IL-7R? blockade increases the expression of immune inhibitory receptor PD-1 on T effector cells, whereas IL-7 inhibits the expression of PD-1 on T cells. PD-1 plays a central role in peripheral tolerance. The role of PD-1 in the pathogenesis of AA has not been yet investigated. In Specific Aim 2 of this proposal, we will first address the role of PD-1 in AA by PD-1 pathway blockade. Lastly, given the therapeutic potential of IL-7 blockade in the treatment for human AA, we will determine the role of IL- 7 in pathogenesis of human AA by determining if serum IL-7 correlates with parameters of disease, by evaluating the IL-7-induced cytokine production in Specific Aim 3. Most importantly, we will define a subset of patients in which the AA scalp transcriptome and IL-7 gene signature because the AA molecular environment is complex and patients with seemingly similar AA presentations have heterogeneous responses to treatment. Overall, the proposed research is strongly aligned with the applicant?s goal of becoming an independent investigator in the field of skin autoimmune diseases. The results of this study will advance our understanding of the pathogenesis of AA, evaluate the role that IL-7 plays in the loss of tolerance in the hair follicle end-organ, and aid us in evaluating therapeutic interventions that target the IL-7/IL-7R pathway in AA.